1. Field of the Invention
This invention relates to improvements in an exhaust gas recirculation system for an internal combustion engine, more particularly to a vacuum control valve (including a stepper motor) for regulating a signal vacuum to be supplied to an exhaust gas recirculation control valve for controlling exhaust gas recirculated back to the engine.
2. Description of the Prior Art
Hitherto, a variety of exhaust gas recirculation systems for recirculating exhaust gas back to the combustion chambers of the engine have been porposed and put into practical use. One of them is constructed and arranged as follows: A diaphragm-operated exhaust gas recirculation control valve is disposed in an exhaust gas recirculation passage which connects an intake air passageway and an exhaust gas passageway of the engine. The diaphragm of the exhaust gas recirculation control valve defines a vacuum chamber to be supplied with a signal vacuum such as an intake vacuum downstream of a throttle valve. The signal vacuum is modified by a diaphragm-operated vacuum control valve which is controlled in response to a control signal output from a control unit. Such an exhaust gas recirculation system is disclosed, for example, in Japanese Patent Publication No. 57-55901 and Utility Model Provisional Publication No. 1-130058.
The vacuum control valve includes a diaphragm defining an exhaust pressure chamber to be supplied with an exhaust gas pressure prevailing in the exhaust gas recirculation passage upstream of the exhaust gas recirculation control valve. The diaphragm further defines an atmospheric chamber to be supplied with atmospheric air or pressure, on the opposite side of the exhaust pressure chamber. A valve member is fixedly mounted on the diaphragm. An atmospheric air introduction port is formed facing the valve member and communicated with a vacuum passage through which the signal vacuum is supplied to the vacuum chamber of the exhaust gas recirculation control valve. Accordingly, when the exhaust gas pressure in the exhaust pressure chamber of the vacuum control valve lowers, the valve member separates from the atmospheric air introduction port, so that atmospheric air is introduced into the vacuum passage. In contrast, when the exhaust gas pressure rises, the valve member closes the atmospheric air introduction port, thereby interrupting introduction of atmospheric air into the vacuum passage. Thus, the opening degree of the exhaust gas recirculation control valve is basically determined by the relationship between the signal vacuum and the exhaust gas pressure, thereby mechanically controlling the characteristics of the amount of exhaust gas recirculated back to the engine combustion chambers.
The vacuum control valve is provided with an actuator which changes the spring load or biasing force of a spring biasing the diaphragm under electronic control of the control unit, thereby electronically controlling the amount of the recirculated exhaust gas at required characteristics. In other words, the spring load of the spring of the vacuum control valve is changed through the actuator, thereby correcting the opening and closing characteristics of the atmospheric air introduction port and accordingly the opening degree of the exhaust gas recirculation control valve.
However, drawbacks have been encountered in the above-discussed exhaust gas recirculation system as follows: During stopping of the engine, the valve member of the vacuum control valve is left seated on a seat defining the atmospheric air introduction port, and therefore it tends to stick to the seat with a sticky material entering the atmospheric air introduction port through the vacuum passage, leaving the atmospheric air introduction port closed. As a result, there is a possibility that the valve member cannot move or lift from its closing position even upon restarting of the engine. This causes a failed control of exhaust gas recirculation, while degrading the driveability of the engine.